This proposed FIRCA project is for epidemiological research on the earliest stages of youthful drug involvement, including coca paste and other cocaine use. The research involves analyses of data from an already completed probability sample survey of more than 45,000 youths 12-21 years of age, who attended public and private schools in Chile during 1999. The survey included standardized assessments of drug involvement, mental health, and other constructs of interest. The project represents a collaboration between Professor James C. Anthony of the Johns Hopkins University School of Hygiene and Public Health (designated principal investigator), and Dr. Luis Caris of the University of Chile School of Public Health (designated foreign collaborator). Potential public health significance of the project is heightened by apparently increased levels of coca paste smoking and other cocaine use by young people in Chile over the past five years, a possible externality or unintended consequence of U.S. policies and programs to reduce cocaine demand and supply within our own borders. As planned, the project will yield new epidemiological evidence on cocaine use by young people, as well as new epidemiological evidence on tobacco smoking, another form of drug-taking of readily apparent public health importance. Innovation associated with this project is enhanced by the new evidence to be gained, and by a focus on the earliest stages of drug involvement (i.e., transitions from first chance to try a drug to actual use of the drug). The team will use methods not often seen in epidemiological analyses of school survey data, e.g., multivariate analyses and methods to study clustering of youthful drug use within schools and communities. With respect to coca paste, cocaine, and tobacco, the FIRCA research team will estimate clustering within schools and communities, derive provisional estimates of the recent incidence of drug involvement, prepare and submit scientific articles on these results, and prepare and submit RO1 proposals to enhance and sustain the international collaborative research. The proposed work will enhance the U.S. RO1 project and will strengthen the epidemiology research capacities of the University of Chile research team.